Integrating indirect-conversion X-ray detectors can be used in X-ray imaging, for example in computed tomography, angiography or radiography. The information contained in the local modulation of the incident X-rays is detected by the X-ray detector, wherein the X-ray detector converts the information into a digital signal. In indirect-conversion X-ray detectors, the X-rays or the photons can be converted into light by a suitable converter material and into electric pulses by means of photodiodes. The converter material used is frequently scintillators, for example GOS (Gd2O2S), CsJ, YGO or LuTAG.
A scintillator unit comprises the converter material. Scintillators are in particular used in medical X-ray imaging in the energy range up to 1 MeV. Typically, so-called indirect-conversion X-ray detectors, so-called scintillator detectors, are used with which the conversion of the X-rays or gamma rays into electric signals takes place in two stages. In a first stage, the X-ray or gamma quanta are absorbed in a sub-region of the scintillator unit and converted into optically visible light, a quantity of light—this effect is known as luminescence. The light excited by luminescence is then converted in a second stage into an electric signal by a first photodiode or photomultiplier optically coupled to the scintillator unit in a sub-region of an evaluation unit, read-out via evaluation electronics or readout electronics and then forwarded to a computing unit.
Scintillators, for example cesium iodide deposited from the gas phase, can have anisotropic light conduction. Cesium iodide has a needle-shaped structure. As a result of this anisotropic light conduction, the quantity of light is not propagated laterally and hence the incident X-rays' positional information is retained. However, cesium iodide is not suitable for all modalities in medical imaging. For example, it is unsuitable for computed tomography since it neither has neither the necessary absorption properties nor a sufficiently stable signal response.
Suitable ceramic scintillators, such as, for example gadolinium gallium aluminum garnet (GGAG), comprise isotropic light conduction. It is also possible to use GOS as a ceramic scintillator. To retain the incident X-rays' positional information, the scintillator unit has a complicated structure. Generally, sawing processes are used to separate volume units, which define the pixels of the X-ray detector. The interstices can be filled with a reflecting material.
The sub-regions of the scintillator unit and the evaluation unit are as a rule subdivided such that a sub-region of the evaluation unit is assigned to each sub-region of the scintillator unit. This is then called a pixelated X-ray detector. X-ray detectors, such as those used in computed tomography, for example, are typically constructed from a plurality of modules comprising a stray radiation grid, a scintillator unit, an evaluation unit with photosensors or photodiodes, for example as a photodiode array, and with electronic units for converting the analog signals into digital information and a mechanical support. The stray radiation grid is used to suppress stray radiation. The mechanical support is used to assemble the stray radiation grid, the scintillator unit and the evaluation unit. The stray radiation grid, scintillator unit and photodiode are typically pixelated in the same way in two directions, for example into rectangular or quadratic pixels. In order to achieve good dose utilization with a simultaneously low degree of crosstalk between the pixels, the stray radiation grid, scintillator unit and photodiode are positioned very precisely with respect to one another when constructing the modules.
Known from WO 2009008911 A2 is a bundle of drawn fibers that have unagglomerated nanocrystallite scintillation particles in glass or plastic cores with a maximum spacing of 0.1 μm. The bundle of drawn fibers also comprises a cladding with X-ray absorbing mixtures in the cladding composition. Optionally, covering the bundle can prevent light from emerging at the X-ray incidence side while the X-rays are able to pass into the fiber core. To image the light exiting the fiber bundle at submicron intervals, it is preferable to use light expansion by way of a lens system or a fiber bundle expander. However, due to the numerous optical interfaces formed by the embedding of the scintillator particles in the plastic or glass matrix of the fiber core, a large portion of the light leaves the fibers due to the scatter. In addition, light conduction by longer fibers such as those required for the absorption of typical X-ray energy in computed tomography are no longer possible.